This invention relates to a solid-state laser device which is capable of converting a fundamental wavelength into a harmonic wavelength to generate, as an output laser beam, a laser beam having the harmonic wavelength.
A conventional solid-state laser device of the type described has been proposed in an article which is contributed by Lu Baosheng et al to Chinese Physics Letters Vol. 3, No. 9, pages 413-416 (1986) and which is entitled "Excited Emission and Self-Frequency-Doubling Effect of Nd.sub.x Y.sub.1-x Al.sub.3 (BO.sub.3).sub.4 Crystal". More specifically, the solid-state laser device comprises, as a solid-state laser medium, a nonlinear optical crystal which is represented by Nd.sub.x Y.sub.1-x Al.sub.3 (BO.sub.3).sub.4 and which thus includes Nd. Inasmuch as Nd is operable as a laser activator, the above-mentioned nonlinear optical crystal is capable of emitting a primary laser beam which has a fundamental wavelength determined by Nd when the nonlinear optical crystal is excited or pumped by an excitation beam. The excitation beam may be referred to as a pumping beam. The primary laser beam will be called a fundamental laser beam. In addition, the article reports that wavelength conversion takes place within the nonlinear optical crystal so as to partially convert the primary laser beam of the fundamental wavelength into a subsidiary laser beam of a harmonic wavelength which is derived from the nonlinear optical crystal and that the nonlinear optical crystal is capable of emitting the subsidiary laser beam of the harmonic wavelength. The subsidiary laser beam may be referred to as a harmonic laser beam. Practically, when x is smaller than 0.2, the fundamental wavelength of 1.064 micron meters (.mu.m) is stably converted into a second harmonic wavelength of 0.53 micron meter.
In order to carry out such wavelength conversion, the nonlinear optical crystal is combined in the solid-state laser device with a resonator and an excitor for supplying an excitation beam to the nonlinear optical crystal. In the solid-state laser device, the resonator comprises an output mirror and a reflection mirror so as to provide an outer resonator for the primary laser beam of the fundamental wavelength. Both of the output mirror and the reflection mirror are opposed to both ends of the nonlinear optical crystal with spacings left therebetween, respectively. In addition, the excitor is formed by a dye laser directed to a side surface of the nonlinear optical crystal with a distance left between the side surface and the dye laser.
In the solid-state laser device, each of the output mirror and the reflection mirror has an optical characteristic such that about 100% of the primary laser beam is reflected. In addition, the reflection mirror reflects about 100% of the subsidiary laser beam while the output mirror transmits 80% of the subsidiary laser beam.
Under the circumstances, when the excitation beam is supplied from the dye laser to the side surface of the solid-state laser medium, a resonance path for the primary laser beam of the fundamental wavelength is formed between the output mirror and the reflection mirror. Subsequently, the primary laser beam is partially converted in wavelength into the subsidiary laser beam within the resonance path. As a result, the subsidiary laser beam is transmitted through the output mirror as an output laser beam.
However, the above-mentioned solid-state laser device is disadvantageous in that it is bulky in size and has a short life time because the dye laser is used as the excitor. Furthermore, since a dye should be exchanged from time to time, a lot of labor is necessary for maintenance of the solid-state laser device.
In addition, the resonator is formed by the output mirror and the reflection mirror both of which are distant from the solid-state laser medium. Such a resonator is large in size and therefore results in an increase in size in the solid-state laser device. Moreover, the primary laser beam is objectionably weak in strength because a loss inevitably takes place while the primary laser beam is propagated within a space between the output mirror and the reflection mirror. This results in a reduction of conversion efficiency between the primary and the subsidiary laser beams.